A radiometric technic has been developed for measuring gastric mucosal blood flow using C14-labelled aminopyrine. Close agreement between radiometric and colorimetric technics was found. Using this method an attempt is being made to separate histamine H1 and H2 receptor regulation of mucosal blood flow. Using C14-metiamide we have studied kinetics of mucosal membrane binding of the histamine H2-receptor antagonist to define an apparent receptor. We have found that the membrane fraction which contains the histamine H2-receptor contains an adenylyl cyclase which is unresponsive to histamine but is activated by prostaglandins which also increase the mucosal active transport of sodium. BIBLIOGRAPHIC REFERENCES: Chou, A.C., Jacobson, E.D. and Shanbour, L.L. Electrophysiological responses of the canine gastric mucosa to endotoxin, epinephrine and hemorrhage. Circ. Shock 2:13-20, 1975. Bowen, J.W., Kuo, Y-J., Pawlik, W., Williams, D., Shanbour, L.L. and Jacobson, E.D. The electrophysiological effects of burimamide and 16,16 dimethyl prostaglandin E2 on the canine gastric mucosa. Gastroenterology 68:1480-1484, 1975.